Sound reproducing system



NOV. 1958 I. w. CONRAD SOUND REPRODUCING SYSTEM Filed Feb. 1, 1954 AMP! /F/[/? INVENTOR @wwv Wad (iv/wan),

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United States Patent SUUND REPRODUCING SYSTEM Ivan Willard Conrad, Alexandria, Va.

Application February 1, 1954, Serial No. 407,377

17 Claims. (Cl. 179-1) The present invention relates to sound reproducing systems and has for its principal object to provide a high degree of fidelity of sound production.

Another object of my invention is to provide means for securing high fidelity sound reproduction with relatively simple, inexpensive means which can be added readily to existing sound producing or reproducing equipment.

Still another object of my invention is to provide means for selectively reinforcing sound production at desired points in the frequency spectrum.

Essentially, the invention contemplates securing the desired results through the use of feedback in the sound producing system. The objective of improved fidelity will ordinarily require negative or inverse feedback, whereas the objective of selective reinforcement will require positive or regenerative feedback. At the present time one commonly used sound producing system comprises an amplifier of some kind, as for example, an electron vacuum tube amplifier, driving a transducer element, as for example, a loud speaker, by means of which combination energy fed into the system is converted into the physical motion constituting sound or sound-like energy. In the present state of the art the response characteristic of the amplifier portion of such a system has been brought to a high degree of perfection. However, I have observed that even with an amplifier possessing substantially distortionless response as determined by tests for amplitude, phase, and intermodulation response, when such an amplifier is connected to a loud speaker the resulting sound production characteristic reveals pronounced peaks and valleys, thus, to a substantial degree, defeating the distortionless characteristic of the amplifier. In seeking a solution to this problem I have discovered that, if an inverse feedback signal is derived directly from the physical mo tion of the sound producing transducer element, and if such feedback signal is then fed into the system at an earlier point in the sound production process, thereby effectively including the transducer within the feedback loop, the overall fidelity of sound production of the sys tern is materially improved as judged both by subjective listening tests and by physical measurement of the degree Accordingly, the present invention involves effectively including the transducer element of a sound producing system within a feedback loop in the system.

With the foregoing objects in view the invention consists in the novel combinations and arrangements of features as will be hereinafter more fully described, i1lustrated in the accompanying drawings, and defined in the appended claims.

In the accompanying drawings wherein are illustrated different, non-limiting, practical embodiments of the invention, and wherein like characters of reference denote corresponding parts in related views:

Figure 1 is a diagrammatic view illustrating the gen- "eral principle of the invention;

2,860,183 Fatentecl Nov. 11, 1358 embodiment of the invention wherein the sound producing transducer element is a conventional electrodynamic loud speaker and the feedback signal is derived from an auxiliary winding on the voice coil bobbin;

Fig. 3 is a diagrammatic view of another practical embodiment wherein the sound producing element is an electromagnetically driven piston and the feedback signal is derived from a piezoelectric element mechanically coupled to said piston;

Fig. 4 is a diagrammatic view of a third practical embodiment wherein the sound producing element is a magnetic diaphragm driven by an electromagnet in a manner commonly employed for earphone construction, and wherein the feedback signal is derived from an electromagnetic pickup located in close proximity to the driven diaphragm; and

Fig. 5 is a diagrammatic view of still another practical embodiment wherein the sound producing transducer is a piezoelectric unit and wherein the feedback signal is derived from a second set of electrodes coupled to the same piezoelectric element.

Referring to the drawings in detail, there is shown at A in Fig. 1, an amplifier having input leads 10 and 11, and output leads 12 and 13, respectively. Output leads 12 and 13 in turn are connected to transducer T, comprising stator 14 and driven movable armature element 15. Mechanically coupled to driven element 15 is feedback transducer F, having feedback leads 16 and 17 connecting output of feedback transducer F into input leads 11 and 10, respectively. Thus upon receipt by amplifier A of a given signal from any appropriate source S, as for example, from a phonograph pickup or a microphone, the amplified signal including distortion and intermodulation products is impressed upon transducer T and thereby converted into physical motion of driven element 15, which in turn communicates the physical motion to any medium with which it may be in contact, as for example to air. The motion of element 15 in general will not follow accurately and faithfully the signal impressed upon transducer T, because of the effects of inertia, elasticity, resonance, and related factors. Thus in addition to the distortion introduced by amplifier A there will be present in the motion of element 15, still further distortion occasioned by the non-uniform response of transducer T. This composite motion of element 15 will be conveyed, by virtue of mechanical coupling, to feedback transducer P, which is selected to provide in response to mechanical activation a signal of the same general nature as that for which amplifier A is designed. Accordingly, by virtue of the interconection through leads 16 and 17, the signal received by amplifier A from source S will be modified by the signal produced by feedback transducer F. Inserted in the feedback path 16 and 17 there is shown at V a variable amplitude-frequency filter, and at P a variable phasing network, both of types well known in the art, whereby phase and amplitude changes at selective regions in the frequency spectrum may be introduced in the signal being fed into amplifier A from transducer F.

The phase of movement of driven or armature element 15 under certain conditions is shifted with relation to the driving energy received over leads 12 and 13%, particularly in the frequency regions corresponding to mechanical resonant frequencies of element 15; accordingly, V and P provide means whereby the feedback signal can be corrected for any such phase shift and related amplitude variations, thereby insuring that with respect to the signal entering amplifier A from source S, the feedback signal will be of the desired phase and amplitude relation at all points in the useful frequency spectrum. Switches 43 and 44 are provided whereby V and P respectively may be bypassed if desired.

It is noted that the principal mechanical resonances of many acoustical transducers fall within the frequency range 20 to 300 cycles per second; accordingly a preferred, but not a limiting, corrective range for variable networks V and P lies in this approximate frequency region. It is further noted that since at frequenciesslightly lower than resonance, the current through a parallel-resonant circuit has a lagging phase angle, whereas the current through a series-resonant circuit has a leading phase angle, the use of tuned electrical circuits resonant at substantially the mechanical resonance frequencies of the output transducer T offer a preferred, but not limiting, type of electrical element for variable networks P and V.

Under conditions of phase opposition, or inverse feedback, the correction applied to the amplifier by the signal from F will be such as to reduce not only the distortion introduced by amplifier A but also the distortion introduced by non-uniform response of transducer T, thereby rendering the physical motion of driven element more nearly an accurate translation of the signal originating from source S. On the other hand, under conditions of in-phase or regenerative feedback, the physical motion of driven element 15 may be increased in a controlled manner as desired.

Referring to Fig. 2, there is shown diagrammatically a detailed practical embodiment of a transducer T and feedback transducer F suitable for use in the general system illustrated in Fig. 1. In this embodiment, stator 14 of transducer T consists of the field structure 18 of a conventional electrodynamic loudspeaker. Movable element 15 consists of the loud speaker cone 19 together with its associated voice coil bobbin 20 and voice coil 21. At 23 is represented a conventional cone-suspension spider. Leads 12 and 13 extend from voice coil 21. Feedback transducer F in this embodiment consists of an auxiliary coil 22 wound'on voice coil bobbin 2t} and provided with leads 16 and 17. Mechanical coupling between driven element 15 and coil 22 is of course derived by virtue of the common bobbin 20. Thus, upon excitation of voice coil 21 over leads 12 and 13, there is created by virtue of the resulting motion of coil 22 through the magnetic field of stator 18, a potential at 16 and 17 which is available for feedback purposes and which, if applied as shown in Fig. 1, effectively includes the speaker within the feedback loop.

Referring next to Fig. 3, there is shown an alternative practical embodiment of a transducer T and feedback transducer F suitable for use in the circuit of Fig. 1. In this embodiment stator 14 consists of the field structure 24 of a conventional electrodynamic reproducer of the so-called piston type. The driven element 15 in this embodiment consists of a movable piston 25 together with its associated voice coil 26 having connecting leads 12 and 13. Piston 25 is mechanically supported by spider 45. Mechanically coupled to piston 25 by mechanical linkage 27 is a piezoelectric element 28 which together with its associated electrodes 29 and 30 comprises feedback transducer F. Feedback leads 16 and 17 are connected to electrodes 29 and 30, respectively. Thus, upon activation of voice coil 26 over leads 12 and 13, the resulting mechanical motion of sound-producing piston 25 produces a corresponding strain in piezoelectric element 28. The piezoelectric potential thereby created on electrodes 29 and 30 is available through leads 16 and 17 for feedback purposes and, if applied as shown in Fig. 1, cffectively includes the transducer T within the feedback loop.

In Fig. 4 is shown diagrammatically still another alternative practical embodiment of a transducer T and feedback transducer F suitable for use in the circuit depicted in Fig. 1. In this embodiment, the stator 14 consists of a magnetic field structure 31 of a type commonly used for magnetic earphone reproducing equipment. On this magnetic field structure is mounted the driving coil assembly 32 having leads 12 and 13. Movable element 15 consists of a magnetic diaphragm 33 mounted adjacent to field 31. Feedback transducer F comprises an auxiliary magnet 34, associated feedback coil 35 with leads 16 and 17, and that portion of the diaphragm 33 which is immediately adjacent to magnet 34, magnet 34 being so placed that movement of diaphragm 33 will vary the magnetic flux flowing through coil 35. In this embodiment the mechanical coupling between movable element 15 and transducer F is provided by that portion of diaphragm 33 which is common to both. Thus, upon activation of driving coil 32 over leads 12 and 13, diaphragm 33 is caused to move in the usual manner. As a result of such motion, there is induced by the varying magnetic flux through coil 35 a potential at leads 16 and 17 which is available for feedback purposes, and which when applied as shown in Fig. 1, effectively includes the transducer T within the feedback loop.

In Fig. 5 is shown yet another alternative practical embodiment of a transducer T and feedback transducer F suitable for use in the general system illustrated in Fig. 1.

In this embodiment, transducer T is a conventional piezoelectric reproducer. The stator 14 consists of mechanical support 36 to which is affixed a piezoelectric driving element 37. To the movable end of said piezoelectric element 37 there is connected the sound-producing diaphragm or cone 38 which, together with the movable portion of element 37, constitutes the driven element 15 of transducer T. Driving electrodes 39 and 40 are connected to leads 12 and 13, respectively. Mounted on the movable portion of piezoelectric element 37 are feedback electrodes 41 and 42, so positioned as to derive a piezoelectric potential from the strained element 37; electrodes 41 and 42 are connected to leads 16 and 17, respectively. Electrodes 41 and 42 together with the movable portion of element 37 affecting said electrodes constitute the feedback transducer F. The mechanical coupling between transducer F and driven element 15 is, of course, inherent in the use of a common member, namely, the movable portion of element 37. Thus, upon excitation of piezoelectric element 37 by an applied signal over leads 12 and 13, mechanical movement of the movable portion of said element 37 and associated cone 38 will result in a well-known manner. This movement will in turn, by piezoelectric action, induce on electrodes 41 and .42 a potential which is available over leads 16 and 17 for feedback purposes, and which, if connected as shown in Fig. 1, will effectively include the piezoelectric transducer T within the feedback loop.

While only certain specific embodiments of the invention have been illustrated and described to convey the general concept of the invention, it is to be understood that the same is readily capable of various other embodiments within its spirit and scope as defined in the appended claims.

It is further to be understood that although intended primarily for sound producing transducers in air such as loudspeakers, earphones, and the like, the invention is equally applicable to the production of physical motion or vibration in any medium such as in water, metals, etc.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. A sound reproducing system comprising amplifier means having input and output connections, a signal source connected to said input connections, output transducer means connected to said output connections of said amplifier means, and feedback means connecting said output transducer means within a feedback loop of the system, said feedback means including a separate feedback transducer mechanically coupled to said output transducer means, the output of said feedback transducer being connected to an input connection of said amplifier means, and said feedback means further including variable phasing and variable attenuating networks whereby the phase and amplitude of the feedback energy are selectively controlled to vary the system response at selected regions in the frequency spectrum.

2. A sound reproducing system comprising amplifier means having input and output connections, a signal source connected to said input connections, output transducer means connected to said output connections of said amplifier means, and feedback means connecting said output transducer means within a feedback loop of the system, said feedback means including a separate feedback transducer mechanically coupled to said output transducer means, the output of said feedback transducer being connected to an input connection of said amplifier means, and said feedback means further including variable phasing and variable attenuating networks whereby the phase and amplitude of the feedback energy are selectively controlled to correct phase and amplitude distortions generated within said reproducing system, including particularly such phase and amplitude distortions arising from natural resonances of said output transducer means.

3. A sound reproducing system as set forth in claim 2 in which said variable phasing and attenuating networks are connected between said feedback transducer .and said input connections.

4. A sound reproducing system comprising amplifier means having input and output connections, a signal source connected to said input connections, output transducer means connected to said output connections of said amplifier means, and feedback means connecting said output transducer means within a feedback loop of the system, said feedback means including a separate feedback transducer mechanically coupled to said output transducer means, the output of said feedback transducer being connected to an input connection of said amplifier means, and said feedback means further including a variable phasing network whereby the phase of the feedback energy is selectively controlled to correct phase distortions generated within said reproducing system, including particularly such phase distortions arising from natural resonances of said output transducer means.

5. A sound reproducing system as set forth in claim 2 in which the variable frequency-versus-phase response characteristic of the said phasing network is substantially the inverse of the corresponding over-all variable frequency-versus-phase response characteristic of the said amplifying means, said output transducer means, and said feedback transducer in combination, thereby insuring that the feedback energy delivered to the input of said amplifier means is substantially 180 degrees out of phase throughout the frequency spectrum of the system in relation to the phase of the signal energy delivered to said amplifier means by said signal source; and in which the variable amplitude-versus-frequency response characteristic of the said attenuating network is similar to the corresponding over-all variable amplitude-versus-frequency response characteristic of the said amplifying means, said output transducer means, and said feedback transducer in combination, thereby providing feedback gain at difierent portions of the frequency spectrum in substantially the same ratio as the uncorrected response differs fro-m uniformity.

6. A sound reproducing system as set forth in claim 4 in which the variable frequency-versusphase response characteristic of the said phasing network is substantially the inverse of the corresponding over-all variable frequency-versus-phase response characteristic of the said amplifying means, said output transducer means, and said feedback transducer in combination, thereby insuring that the feedback energy delivered to the input of said amplifier means is substantially 180 degrees out of phase throughout the frequency spectrum of the system in relation to the phase of the signal energy delivered to said amplifier means by said signal source.

7. A sound reproducing system as set forth in claim 2 in which the said variable phasing and attentuating' networks include electrical tuned circuits having resonances at the same frequencies as the natural mechanical resonances of said output transducer means, whereby a leading phase angle in the output transducer response near a mechanical resonance frequency of said output transducer means is cancelled in the feedback energy by an equivalent lagging phase angle of said electrical tuned circuits, both phase angles becoming zero :at the resonant frequency.

8. A sound reproducing system as set forth in claim 4 in which the said variable phasing network includes electrical tuned circuits having resonances at the same frequencies as the natural mechanical resonances of said output transducer means, whereby a leading phase angle in the output transducer response near a mechanical resonance frequency of said output transducer means is cancelled in the feedback energy by an equivalent lagging phase angle of said electrical tuned circuits, both phase angles becoming zero at the resonant frequency.

9. In combination, an electromagnetic signal source; amplifier means having an input connected to said signal source for amplifying signals from said source; output transducer means connected to and driven by said amplifier means, whereby amplified signals from said source are converted into mechanical motion; feedback means including a separate feedback transducer deriving electro-magnetic feedback energy solely from said mechanical motion of said output transducer means and applying said feedback energy through variable phasing and attenuating networks to the input of said amplifier means, thereby selectively controlling the amplitude and phase of the feedback energy to correct phase and amplitude distortions generated within the amplifier and output transducer means, including particularly such phase and amplitude distortions arising from natural resonances of said output transducer means.

10. A sound reproducing system as set forth in claim 2 in which said variable phasing network includes tuned electrical circuits resonant within the frequency range 20 to 300 cycles per second.

11. A sound reproducing system as set forth in claim 4 in which said variable phasing network includes tuned electrical circuits resonant within the frequency range 20 to 300 cycles per second.

12. A sound reproducing system as set forth in claim 9 in which said variable phasing network includes tuned electrical circuits resonant with the frequency range 20 to 300 cycles per second.

13. A sound reproducing system comprising amplifier means having input and output connections, a signal source connected to said input connections, output transducer means connected to said output connections of said amplifier means, and feedback means connecting said output transducer means within a feedback loop of the system; said output transducer means including a fixed stator and a movable armature; and said feedback means including a separate feedback transducer coupled to said movable armature for converting the motion of said armature into energy of the same kind as that supplied by said signal source, the output of said feedback transducer being connected to an input connection of said amplifier means; and said feedback means further including variable phasing and attenuating networks whereby the phase and amplitude of the feedback energy are selectively controlled to correct phase and amplitude distortions generated within said reproducing system, including particularly such phase and amplitude distortions arising from natural resonances of said output transducer means.

14. A sound reproducing system as set forth in claim 2 in which said output transducer means comprises an electrodynamic loudspeaker having a fixed magnetic field and a movable coil bobbin located within said field; and in which said feedback transducer comprises an auxiliary winding mounted on said bobbin, whereby motion of said bobbin generates feedback energy in said auxiliary winding.

15. A sound reproducing system as set forth in claim 2 in which said output transducer means includes a fixed magnetic field and a movable coil bobbin located within said magnetic field; and in which said feedback transducer comprises a piezo-electric element coupled to said movable bobbin, whereby feedback energy is derived from said piezo-electric element whenever said bobbin moves.

16. A sound reproducing system as set forth in claim 2 in which said output transducer means includes a magnetic field, driving coils for varying said magnetic field, and a movable magnetic diaphragm positioned within said field and responsive to changes of said field; and in which 15 said feedback transducer comprises an auxiliary magnetic circuit including a polarizing magnetic field, a feedback 8. coil, and a portion of said magnetic diaphragm, whereby motion of said diaphragm varies the magnetic flux in said auxiliary magnetic circuit thereby generating feedback energy in said feedback coil.

17. A sound reproducing system as set forth in claim 2 in which said output transducer means comprises a piezoelectric element loudspeaker; and in which said feedback transducer includes auxiliary electrodes positioned on said piezo-electric element, whereby movement of said piezoelectric element generates feedback energy at said auxiliary electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 1,519,211 Martin Dec. 16, 1924 2,194,175 Wilhelm Mar. 19, 1940 2,285,769 Forster June, 9, 1942 

